There is an extremely high need in society for escaping from dependence on oil and other fossil fuel energy and utilizing sustainable natural energy. Large-sized wind power generation systems are consequently spreading throughout the world.
The regions most suitable for wind power generation are regions at which continuously strong winds can be expected. Offshore wind power generation is also being realized on a global scale. For construction of wind power towers offshore, the base parts of the towers must be driven into the foundation of the seabed. To secure sufficient height of the turbine blades of the wind power towers from the surface of the water, the base parts also have to be of sufficient lengths.
For this reason, the base parts of the wind power towers are tubular structures having plate thicknesses of about 100 mm and large cross-sections of diameters of about 4 m. The overall heights of the towers reach as high as 80 m or more. Assembly of such giant structures on the seashore close to the construction sites simply and efficiently by welding is therefore being sought.
Therefore, in the above way, a never before seen need has arisen for welding extremely thick steel plates of a plate thickness of as much as 100 mm by a high efficiency on-site.
In general, the electron beam welding method is a welding method enabling efficient welding by a high density, high energy beam, but it is necessary to perform the welding in a vacuum chamber while maintaining a high vacuum state, so in the past the size of the weldable steel plate was limited.
As opposed to this, in recent years, as a welding method enabling efficient on-site welding of extremely thick steel plate of a plate thickness of about 100 mm, reduced pressure electron beam welding (RPEBW) has been developed and proposed by the Welding Institute of Great Britain (see WO99/16101).
By using this RPEBW method, even when welding large-sized structures like wind power towers, it is expected to become possible locally reduce the pressure of and efficiently weld just the parts to be welded.
However, on the other hand, with this RPEBW method, the welding is performed in a state of a reduced degree of vacuum compared with the method of welding in a vacuum chamber, so the new issue has arisen that securing the toughness of the melted metal part which was melted by the electron beam, then solidified (hereinafter referred to as the “weld metal”) becomes difficult.
To deal with such a problem, in the past, lining the welding surfaces with sheets of Ni or another insert metal and then performing the electron beam welding so as to make the Ni content of the weld metal 0.1 to 4.5% and improve the Charpy impact value and other toughness values of the weld metal is known by Japanese Patent Publication (A) No. 3-248783 etc.
However, when using the RPEBW method for welding, with this method, the Ni and other elements in the insert metal will not uniformly disperse to the heat affected zone. Rather, they will increase the difference in hardness between the weld metal and heat affected zone (hereinafter referred to as the “HAZ”). Therefore, conversely, the problem of a large variation in the toughness of the HAZ surfaced.
In general, as an indicator for quantitative evaluation of the safety of a welded structure, the fracture toughness value δc based on fracture dynamics found by a CTOD test is known. A welded joint obtained by welding by the conventional RPEBW method has a large variation in the toughness of the heat affected zone, so it was difficult to secure a sufficient fracture toughness value δc.
On the other hand, to secure the fracture toughness value Kc in an electrogas welded or other large heat input welded joint, the method of controlling the hardness ratio of the weld metal and the base material to 110% or less to improve the fracture toughness Kc of the boundary (hereinafter also referred to as the “FL”) between the weld metal and the base material is disclosed in Japanese Patent Publication (A) No. 2005-144552.
However, securing the fracture toughness value δc of the electron beam welded joint requires that both the FL and the weld metal be satisfactory in fracture toughness value δc. If, in the same way as with a large heat input welded joint, the hardness of the base material is reduced to 110% or less, the problem will arise that the fracture toughness value of the weld metal at the electron beam welded joint will not be able to be secured.
Further, the electron beam welding method is a method of using the energy of an electron beam to melt once, resolidify, then weld the base material of the weld zone. It is difficult to control the hardness of the weld metal, the fracture toughness value δc, and other characteristics by the welding wire etc. as easily as like with electrogas welding or other large heat input arc welding.